Average Atomic Mass Calculator

Calculate the weighted average atomic mass of an element using isotope data.

Enter isotope masses and abundances to find the average atomic mass. Supports multiple isotopes and both percent or decimal abundances.

Examples

See how to calculate average atomic mass for different elements.

Carbon (C)

Average

Calculate the average atomic mass of carbon using its two main isotopes.

Isotope Masses: 12.000, 13.003

Isotope Abundances: 98.93, 1.07

Atomic Number (Z): 6

Mass Number (A): 12

Chlorine (Cl)

Average

Find the average atomic mass of chlorine from its isotopes.

Isotope Masses: 34.969, 36.966

Isotope Abundances: 75.78, 24.22

Atomic Number (Z): 17

Mass Number (A): 35

Boron (B)

Average

Calculate the atomic mass of boron using two isotopes.

Isotope Masses: 10.0129, 11.0093

Isotope Abundances: 19.9, 80.1

Atomic Number (Z): 5

Mass Number (A): 11

Magnesium (Mg)

Average

Find the average atomic mass of magnesium using three isotopes.

Isotope Masses: 23.985, 24.986, 25.983

Isotope Abundances: 78.99, 10.00, 11.01

Atomic Number (Z): 12

Mass Number (A): 24

Other Titles
Understanding Average Atomic Mass: A Comprehensive Guide
Master the concept of average atomic mass and isotope calculations.

What is Average Atomic Mass?

  • Definition and Importance
  • Isotopes and Atomic Mass
  • Weighted Average Concept
Average atomic mass is the weighted mean of the atomic masses of an element's naturally occurring isotopes. It reflects both the mass and relative abundance of each isotope.
Why is Average Atomic Mass Important?

Common Element Examples

  • Carbon: 12.000 (98.93%), 13.003 (1.07%) => 12.01 amu
  • Chlorine: 34.969 (75.78%), 36.966 (24.22%) => 35.45 amu

Step-by-Step Guide to Using the Calculator

  • Inputting Isotope Data
  • Understanding Results
  • Checking for Errors
Enter the masses and abundances of each isotope. The calculator multiplies each mass by its abundance, sums the results, and displays the average atomic mass.
How to Enter Data Correctly

Input/Output Examples

  • Input: 12.000, 13.003 | 98.93, 1.07 => Output: 12.01 amu
  • Input: 34.969, 36.966 | 75.78, 24.22 => Output: 35.45 amu

Real-World Applications of Average Atomic Mass

  • Chemistry Education
  • Laboratory Analysis
  • Scientific Research
Average atomic mass is essential in chemistry for stoichiometry, molecular calculations, and understanding element properties. It is widely used in labs and research.
Where is Atomic Mass Used?

Practical Uses

  • Calculating molar mass for reactions
  • Determining isotope ratios in samples

Common Misconceptions and Correct Methods

  • Abundance Format
  • Matching Masses and Abundances
  • Total Abundance
A common mistake is mixing percent and decimal abundances or not matching the number of masses and abundances. Always check your input format and totals.
Avoiding Calculation Errors

Input Mistakes

  • Wrong: 12.000, 13.003 | 0.989 => Error
  • Right: 12.000, 13.003 | 98.93, 1.07 => Correct

Mathematical Derivation and Examples

  • Weighted Average Formula
  • Sample Calculation
  • Interpreting Results
The average atomic mass is calculated as: (mass1 × abundance1 + mass2 × abundance2 + ...)/total abundance. For percent, total should be 100; for decimal, 1.0.
Worked Example

Formula in Action

  • (12.000 × 98.93 + 13.003 × 1.07) / 100 = 12.01 amu
  • (34.969 × 0.7578 + 36.966 × 0.2422) = 35.45 amu